Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 5,602	$ 5,617	$ 11,167	$ 10,783
Net earnings allocated to participating securities	(33)	(25)	(62)	(46)
Net earnings allocated to common stock	$ 5,569	$ 5,592	$ 11,105	$ 10,737
Weighted average common shares outstanding, including shares considered participating securities	14,404	14,450	14,410	14,447
Less: Average participating securities	(80)	(60)	(73)	(56)
Weighted average shares	14,324	14,390	14,337	14,391
Basic earnings per common share	$ 0.39	$ 0.39	$ 0.77	$ 0.75
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 5,569	$ 5,592	$ 11,105	$ 10,737
Weighted average common shares outstanding for basic earnings per common share	14,324	14,390	14,337	14,391
Add: Dilutive effects of assumed exercises of stock options	0	1	0	2
Weighted average shares and dilutive potential common shares	14,324	14,391	14,366	14,393
Diluted earnings per common share	$ 0.39	$ 0.39	$ 0.77	$ 0.75
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,362	$ 2,375	$ 4,731	$ 4,751
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 3,207	$ 3,217	$ 6,374	$ 5,986